1. Field of the Invention
The present invention relates to lubricating oil composition for internal combustion engines, and more particularly to lubricating oil composition for internal combustion engines which is consumed in a reduced amount in internal combustion engines and realizes excellent fuel consumption characteristics (fuel cost) and rubber sealing characteristics. The invention also relates to a method for lubricating an internal combustion engine through use of such an oil composition.
2. Background Art
Lubricating oil, which is used at the interface of the pistons and cylinders of internal combustion engine, is consumed together with fuel during operation of the internal combustion engine. Generally speaking, the consumption of the lubricant oil increases as the engine speed or output power of the internal combustion engine increases. One preferable measure to reduce the consumption of lubricant oil may be to use an oil having as high a viscosity as possible.
In the meantime, when lubricant oil having high viscosity is used, the fuel consumption of internal combustion engines increases due to friction loss and power loss, resulting in an increase of fuel consumption rate. Also, sealing characteristics of rubber may sometimes be degraded.
Under the above circumstances, in order to reduce consumption of lubricant oils and to maintain or improve the sealing characteristics of rubber, a variety of lubricant oils, such as those containing certain types of lubricant base oils for the purpose of adjusting viscosity and those containing a variety of additives, have been developed for use with internal combustion engines and put into practical use.
However, since lubricant oils that have conventionally been developed and used for internal combustion engines do not satisfy all of the aforementioned requirements, there still remains the need for lubricant oil for internal combustion engines, or internal combustion engine oil, which simultaneously satisfies all the above requirements.
The present inventors have conducted careful studies to attain the above object and have found that the object is effectively attained by a composition comprising a base oil and a specified additive, the base oil containing (A) a mineral oil having a specified viscosity and (B) poly-.alpha.-olefin, which is one type of a synthetic oil.
Hitherto, there have been known lubricating oil composition for internal combustion engines which is made up of one or more species of mineral oils in admixture (for example, Japanese Patent Application Laid-Open (kokai) No. 64-6094), and another type of lubricating oil composition for internal combustion engines which contains poly-.alpha.-olefin (B) in combination with other polymers (for example, Japanese Patent Application Laid-Open (kokai) No. 59-89397). With the state of the art being as described above, the present inventors have found that use of a specified mineral oil (A) and a poly-.alpha.-olefin (B) in combination can effectively attain the aforementioned goals, i.e, reduction in the consumption of the engine oil, improvements of fuel consumption rate, etc. The present invention was accomplished based on this epoch-making finding.